Apparatus for charging a continuous digester



June 11, 1968 N. ILANDER 3,388,038

APPARATUS FOR CHARGING A CONTINUOUS DIGESTER Filed Dec. 7, 1964 INVENTUR )VL' Zs IZancZer ATTORNEYS Unite 3,388,038 APPARATUS FOR QHARGENG A CONHNUOUS DHGESTER Nils Ilander, Linkoping, Sweden, assignor to Alrtiebolaget N ordiska Armaturfahrikerna, Linlroping, Sweden, a limited company of Sweden Filed Dec. 7, 1564, Ser. No. 416,338 Claims priority, application Sweden, Dec. 6, 1963, 13,558/ 63 1 Claim. (Cl. 162-246) ABSTRAQT F THE DISCLOSURE The present invention relates to methods and apparatus for charging a lignin-containing fibrous material, such as wood chips, into a continuously operating digester, with the possibility of pretreating this material under conditions which both physically and chemically are different from the process conditions in the digester.

It is known that a complete impregnation of lignincontaining fibrous material may be of great importance for a uniform pulping of the fibrous material and permits a practically instantaneous transfer of the material to a zone of maximum reaction temperature. It is also known that pretreatment of the fibrous material with a liquid of a chemical composition different from that of the digesting liquid (multistep digestion) is of great importance for both pulp type and grade. According to the present invention, pretreatment or impregnation of the fibrous material can be carried out independently of the conditions in the digester, including the possibility of varying the physical conditions (pressure, temperature, time) as well as the chemical conditions (chemical composition and con centration) With hitherto used charging devices (rotating valves, metering rums etc.), it is not possible in practice to pretreat cellulosic raw material in the metering device itself due to the fact that a sufiicient time for impregnation is not available, but any impregnation has to be carried out in the digester and use of pretreatment liquids of a chemical composition different from that of the digesting liquid is possible only with special arrangements.

According to the present invention, the lignin-containing raw material and a pretreating liquid are introduced into a charging container which is brought to the desired pressure and prior to introduction of the material into the digester is brought to the same pressure as the digester by means of a cooking liquid circulation system, whereupon the pretreated material is introduced into the digester by the cooking liquid circulating in the circulation system. To enable a sufiicient treating time in the container to be reached, it is suitable to use two or more charging containers arranged in parallel and adapted to be alternatingly placed in the circulation system, so that one container can 'be used for feeding the raw material to the digester while raw material is charged into one or more other containers and pretreated therein.

For charging the pretreated material, there is arranged in the charging container a feeding device of known type, e.g. a screw, which feeds the material toward a portion States Patent 0 3,388,@38 Patented June 11, 1868 "ice of the charging container through which the circulating digesting liquid flows.

It is known that heat transfer from liquid to liquid is very slow without efiicient agitation (circulation). In accordance with the present invention, the outlet of the feeding device is preferably so formed that no substantial admixture of the p-retreating liquid and the flowing digesting liquid occurs. The raw material is passed by the feeding device from the pretreating liquid temperature region to the digesting liquid temperature region at a rate such that heat transfer in a direction opposite to that of the feed of the container contents cannot take place. Corresponding to the amount of raw material carried from the charging container to the digester, pretreating liquid is continuously introduced into the container. Thus, the temperature in the charging container is independent of the temperature of the digesting liquid.

By a suitable selection of the size of the container, the time of pretreating under pressure in the liquid phase can be predetermined as desired.

As stated above, the pretreating liquid may be of a composition different from that of the digesting liquid.

The invention will be described more in detail with reference to the accompanying drawing, which diagrammatically illustrates a digester plant including a charging system in accordance with this invention.

The digester 1 comprises an elongate vertical tube which at its lower end has means 2 for withdrawing the pulped material to the outlet conduit 3. Through a screen 4, digesting liquid is withdrawn via the conduit system 5 with valves 6 by a circulating pump 7 to the heat exchanger 8 where the digesting liquid is heated by steam suplied through the conduit 9 with valve 10, the resulting steam condensate being withdrawn through conduit 11. The heated digesting liquid is returned to the digester top through conduit 12, three-way valve 13, conduits 14, or conduits 16, 17, three-way valve 18 and conduit 19.

Charging of wood-chips and pretreatment thereof are carried out by the aid of two charging containers 20, 21 which have each an outlet portion 22 and 23, respectively, interposed between the conduits 14, 15 and 16, 17, respectively, so that the circulating digesting liquid flows through the outlet portion 22 or 23 depending on the setting of the three-way-valves 13, 18. In the condition illustrated, the valves 13, 18 are set so that the digesting liquid flows from the valve 13 through conduit 14. outlet portion 23 of container 21 and conduit 15 to valve 18.

Each charging container is also connected to a chip conduit 24 and 25, respectively, having a suitable valve means 26, 27, preferably a ball valve. The ball valve 26 of the container is open so that wood chips can enter this container which as mentioned is disconnected from the digesting liquid circulation. The ball valve 27 of container 21, however, is closed, and this container is operating to feed chips into the digester through the digesting liquid circulation conduit. To this end, the container 21 is provided with a screw feeder 28 in its bottom portion above a screen bottom 29. The screw feeder 28 extends partly into the outlet portion 23 but not up to the mouths of the conduits 16, 17 so that a free space is formed in front of the screw end. The container 20 has a similar screw 30 and screen bottom 31. The screws are driven by motors which are not shown.

As seen from the above, one container is filled with chips while the other serves to charge chips into the digester, the screw 28 feeding the chips into the outlet portion 23 where they are entrained by the flowing digesting liquid to the conduit 19 and thus enter the digester 1 together with the liquid.

After the charging container has been filled with chips and before they are charged to the digester, pretreatment with an impregnating liquid is carried out in the charging container, said impregnating liquid being supplied through a condut 37 and a branch conduit 38, which conduits have valves 39, 40 and lead to nozzles 41, 4-2 in the chip conduits 24, 25 above the ball valves 26, 27. Outlet conduits 43, 44 with valves 45, 46 extend from the bottoms of the charging containers below the screens 29, 31. Furthermore, conduits 47, 48 are connected to the charging containers below the ball valves, said conduits extending from a three-way valve 49 connected to a conduit 51 extending from a pump 50.

In the case illustrated on the drawing, valves 39, 45, 46 are closed, While valve 40 is open, so that impregnating liquid can be introduced into the container 20 from a tank (not shown) through conduits 37, 38 while charging container 21) is being filled with chips.

Simultaneously, the valve 49 is set so that the pump 51 and conduit 51 communicate with the container 21 through conduit 47, so that impregnating liquid is pumped to this charging container at a rate corresponding to the amount of material charged to the digester. At the same time, the feed of the impregnated cellulosic material by means of feed screw 28 prevents equalization of tempera ture between the impregnating liquid and the digesting liquid.

After the container 20 has been filled, with chips and impregnating liquid, the ball valve 26 and the valve 40 are closed. This container is now ready to serve for charging chips as soon as the container 21 is empty of chips. At this time, valves 13, 18 are set so that the digesting liquid flows through the outlet portion 22 of the container 28, whereby this container is brought to the digester pressure. Simultaneously or after a predetermined time, the feed screw 30 is started to start the charging of chips. Also, the valve 49 is set to introduce liquid into the container 20 at a rate corresponding to the rate of removal of the material.

The charging container 21 is filled with impregnating liquid under digester pressure. The liquid is removed by opening valve 46, whereby the pressure is also lowered. The ball valve 27 and the valve 40 can now be opened and the charging container 21 is filled with chips, which is simultaneously impregnated therein.

Fresh digesting liquid is supplied to the digester through the conduit 57 with valve 58.

As seen from the above, the arrangement according to this invention provides a possibility of carrying out, with mechanically simple means, a completely continuous charging of chips to a continuously operating digester and at the same time it enables a pretreatment of the chips to be carried out in a simple manner With an impregnating liquid of a composition different from that of the digesting liquid and under conditions diflering from those prevailing in the digester.

The invention is not limited to the embodiment illustrated and described. For example, more than two charging containers may be used, which are arranged to be connected one or more at a time to the circulation system. It is also pointed out that in some cases it may be suitable to leave a certain amount of pretreated chips in a charging container before the next chip filling step, in order to be able to start the charging of chips at an earlier time from this container and yet obtain sutlicient time for impregnating the freshly supplied chips under pressure.

I claim:

1. A system for charging cellulosic material to a continuously operating digester, comprising a charging container, a screw feeder horizontally positioned in the bottom of the charging container, a digester liquid recirculation conduit, means for introducing cellulosic material into the charging container, means for feeding treating liquid into the first-mentioned means, and means clefining an outlet for the screw feeder connected to the digester liquid recirculation conduit so that the impregnated cellulosic material is admixed with the digesting liquid in said conduit without substantial admixture of the treating liquid in the charging container and said digesting liquid.

References Cited UNITED STATES PATENTS 2,996,422 8/1961 Durant et al. 162-19 3,041,232 6/1962 Richter et al. 162-17 3,180,789 4/1965 Fuchiwaki 162-17 S. LEON BASHORE, Primary Examiner. 

